New technology to fabricate semiconductor films on polymer substrates are needed in electronic industry for many applications. Semiconductor materials are particularly useful, if they can be crystallized with good structural integrity on polymer substrates so that the films can be used to function in electronic circuits, light detectors, light emitting diodes (LED), thin film transistors (TFT), CMOS and SRAMS. These have a variety of end uses as for example solar cells, display devices and electronic circuits.
One approach to making crystallized semiconductor films on any substrate is to deposit the semiconductor film on the substrate and anneal the semiconductor film using a pulsed laser source. Many have used laser annealing to achieve low temperature processing in making thin film transistors on glass substrates. See for example Kyung Ha Lee et al., IEEE Electron Device Lett. 17, 258 (1996). On polymer substrates this approach is difficult because the laser can heat the polymer substrate and damage both the polymer/semiconductor interface and the semiconductor film even when thermally insulating layers are introduced in between the polymer substrate and the semiconductor film. The polymer substrate must also be stable to changes in ambient moisture (humidity) and temperature. Resistance to elevated temperatures, as well as high mechanical strength, impact resistance and chemical resistance are also all desirable.
Various prior techniques for improving the crystallinity of semiconductor films by laser annealing and doping the films are exemplified by U.S. Pat. No. 4,059,461 issued Nov. 22, 1977 to J. C. C. Fan et al.; U.S. Pat. No. 4,309,225 issued Jan. 5, 1982 to J. C. C. Fan et al.; U.S. Pat. No. 4,400,715 issued Aug. 23, 1983 to S. G. Barbes et al.; U.S. Pat. No. 4,719,183 issued Jan. 12, 1988 to M. Maekawa; U.S. Pat. No. 4,751,193 issued Jun. 14, 1988 to J. J. Myrick, U.S. Pat. Nos. 5,346,850 issued Sep. 13, 1994 and 5,538,564 issued Jul. 23, 1996 to J. L. Kaschmitter et al. Various prior techniques for reducing the polymer substrate CTE are exemplified by U.S. Pat. No. 5,552,210 issued Sep. 1996 to Horn et al.; and U.S. Pat No. 5,739,193 issued Apr. 14, 1998 to Walpita et al.
In these prior developments and research efforts laser annealing have been done to improve crystallinity of crystalline or amorphous semiconductors, particularly silicon, deposited on insulator films on substrates such as PET, PES, PTFE at a range of processing temperatures from room temperature to 180.degree. C. Semiconductor films deposited and laser annealed on deposited films of metal and oxide on these plastics does not retain film integrity under temperature cycling and during use over short and long time periods because large mismatch of the coefficient of thermal expansion (CTE) between the deposited films and the plastic substrates in the operating temperature ranges. Therefore the laser annealing technologies can not be effectively utilized for plastic substrates in practical applications.
The present invention permits the fabrication of inexpensive yet highly efficient electronic devices, such as photovoltaic cells, on the filled substrates by the use of laser annealing techniques.